Hydraulic fracturing is generally used to stimulate production of hydrocarbons from hydrocarbon wells. Hydraulic fractures are created in subterranean formations by injecting high viscosity fluid (also referred to as fracturing fluid) at a high flow rate into well boreholes. The tensile fractures thus-created can be about 100 m long. The fracturing procedure generally takes from about 30 minutes to 4 hours.
In order to create a high hydraulic conductivity drain in the formation, the fracturing fluid usually contains proppant, small particles which are added to the fluid to keep the fracture open once the injection is stopped and pressure is reduced. These particles can be sand grain or ceramic grains. The width of the fracture during propagation is about 1 cm, and 4 mm once closed on proppant.
To be efficient, the fracture should be contained within the reservoir formation and not propagate into the adjacent layers. It should also be of sufficient length and width. Evaluation of the geometry of the fracture is therefore an important step to ensure treatment optimization.
Fracture geometries can be evaluated utilizing various techniques and methodologies. The mostly widely used is a method of indirect evaluation based on analysis of the pressure response during the fracture treatment and subsequent production. The method is described, for example, in Reservoir Stimulation, Third Edition, M. J. Economides and K. G. Nolte (Ed.), Chichester, UK, Wiley, (2000). This approach provides, however, only very general information about fracture length and fracture width and does not provide any information about the exact fracture geometry. More reliable acoustic fracture imaging technology for field applications can be based on event location using passive acoustic emission. Such technology is described, for example, in A practical guide to hydraulic fracture diagnostic technologies, by D. Barree, M. K. Fisher and R. A. Woodroof, paper SPE 77442, presented at the SPE Annual Technical Conference and Exhibition held in San Antonio, Tex., USA, 28 Sep. to 2 Oct. 2002. Acoustic emission events generated by micro-earthquakes around the fracture during hydraulic fracturing are recorded by an array of geophones or accelerators placed in adjacent boreholes. The micro-earthquakes may be caused by the high stress concentration ahead of the fracture or by the decrease of effective stress around the fracture following fracturing fluid leak-off into the formation. In the best cases, the events can be analyzed to provide some information about the source mechanism (energy, displacement field, stress drop, source size, etc.). However, they do not provide direct quantitative information on the fracture. The approach is commonly used in the field and is particularly suited for the estimation of fracture azimuth and dip, but not for an accurate determination of the position of the fracture tip. Another disadvantage of the approach is that the micro-earthquakes are spread around the fracture and produce a cloud of events, which do not allow a precise determination of the fracture geometry.
Yet another technique for evaluating hydraulic fracture shapes is tiltmeter mapping, also discussed in the paper by D. Barree, et al. referenced above. This technique involves monitoring the deformation pattern in the rock surrounding the fracture. An array of tiltmeters measures the gradient of the displacement (tilt) field versus time. The induced deformation field is primarily a function of fracture azimuth, dip, depth to fracture middle point and total fracture volume. The shape of the induced deformation field is almost completely independent of reservoir mechanical properties and formation stress state, if the rock is homogeneous.
Disadvantages of this technique are first of all in that surface tiltmeters cannot accurately resolve fracture length and height due to the depth of the fracture below the surface since the measurement distance is large compared to the fracture dimensions. Although downhole tiltmeters placed in the treatment borehole can provide better information on the fracture height they still cannot resolve the fracture length.
The above-mentioned techniques are based on observation of perturbations of mechanical fields, such as a reservoir pressure field and a rock deformation field caused by a hydraulic fracture. The present invention relies on perturbations of the electric and magnetic fields generated due to the electrokinetic effect by a fluid flow within a reservoir in a vicinity of a hydraulic fracture.
The electrokinetic effect is a fundamental physico-chemical phenomenon of generation of an electric current by the fluid flow through a permeable medium, or through a narrow channel, such as a fracture. Its primary cause is the difference in mobility of ions, some of which are fixed at the surface of the solid skeleton (matrix) of the porous medium, or the channel walls, while counter-ions in solution can move with the pore fluid, (or force it to move, if an electric field is applied; such a flow is referred to as electroosmotic flow, see for example Coelho, D., M. Shapiro, J. F. Thovert, and P. M. Adler, “Electro-osmotic phenomena in porous media”, J. of Colloid and Interface Science, 181, 169-190, 1996).
This approach is very different from approaches which rely on the injection of an electric current which is generated at the surface and flows through a well casing inside a fracture, as described in U.S. Pat. No. 6,330,914 to Hocking and Wells. In U.S. Pat. No. 6,330,914 a fracture must be opened by a highly conductive fluid (i.e. low electrical resistivity) for the fracture to act as an “electrified sheet”. The flow of a fracturing fluid is therefore intended for creating and propagating the fracture and does not directly relate to the creation of the electromagnetic field itself. The advantage of this method is the creation of a strong electro-magnetic field, but it requires a highly conductive fluid and might be limited in depth due to electric current leakage through the casing.
In U.S. Pat. No. 5,519,322 the effect is used to measure the permeability of a formation penetrated by a borehole by measuring the magnetic field generated by the flow of fluid injected into the formation. The permeability measured in this way provides information on the capacity of the reservoir to produce oils.
A need exists to provide approaches for evaluating shapes of hydraulic fractures in rock formations, which approaches mitigate or even exclude disadvantages and deficiencies explained above.